The exemplary embodiment relates to the printing arts. It finds particular application in connection with optimization of consumable usage, such as the usage of toners in a printing system comprising multiple marking engines, and will be described with particular reference thereto. However, it will be appreciated that the exemplary embodiment finds application in other printing systems.
Electronic image forming systems, such as printing/copying systems, typically employ an input terminal which receives images in digital form and conversion electronics for converting the image to image signals or pixels. The printing system may include a scanner for scanning image-bearing documents or be connected to a computer network which supplies the digital images. The image signals are stored and are read out successively to a marking engine for formation of the images and transfer of the images to a print medium, such as sheets of paper.
In typical xerographic (electrophotographic) printing systems, such as copy machines and laser beam printers, the marking engine includes a photoconductive insulating member, which is charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member, which corresponds to the image areas contained within the document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a marking material. Generally, the marking material comprises toner particles adhering triboelectrically to carrier granules, which is often referred to simply as toner. The developed image is subsequently transferred to the paper. The fusing of the toner image onto paper is generally accomplished by applying heat to the toner with a heated roller and application of pressure. In multi-color printing, successive latent images corresponding to different colors are recorded on the photoconductive surface and developed with toner of a complementary color. The single color toner images are successively transferred to the paper to create a multi-layered toner image on the paper. The multi-layered toner image is then permanently affixed to the paper in the fusing process.
Printing systems have been developed which employ multiple marking engines for providing higher print outputs by distributing a print job among the marking engines. These systems may include several black, process (or full) color, and/or custom color (single color or monochrome) marking engines for printing of selected pages within a print job. Each marking engine has a number of components which can be regarded as consumables, since they are designed to be consumed or worn out and replaced at intervals during the normal lifetime of the printing system. Consumables for xerographic marking engines typically include toner cartridges, photoreceptor belts, and the like. Ink jet printers often have ink cartridges containing ink in liquid or solid form. In a color marking engine for a xerographic process, for example, there may be four toner cartridges, one for each of the cyan, magenta, yellow, and black separations of the image. As the number of marking engines in a printing system increases, the number of times any one of the toner cartridges needs to be replaced in a given time period increases. For example, in a printing system with four process color marking engines, there may be sixteen toner cartridges. Even where each marking engine prints an approximately equal number of pages, differences in the content of the pages and individual attributes of the marking engines can result in unequal consumption of the marking materials. For example, a page with a large proportion of solid black or other color will consume more toner than a page which is light grey or has only a small area of coverage. Since the cartridges tend to run out at different times, it is difficult for the operator to set up a replacement schedule for simultaneous replacement of cartridges or other consumable items without resulting in considerable wastage of the consumables. An alternative is to allow one or more of the cartridges to run out, with print jobs being reallocated to marking engines which have residual marking media. However, this can lead to a reduction in productivity of the system until the empty cartridges are replaced.